Foundation, Concrete and Earthquake Engineering

Engineering Properties of Soil in Relation to Consistancy Limits

Definition of Consistency Limits:

The water content at which the soil changes from one state to other is known as consistency limits or Atterberg’s limits.

Atterbarg define these state as

  • liquid state
  • plastic state
  • Semi-solid state
  • Solid state

These limits are:

  • Liquid limit
  • Plastic limit
  • Shrinkage limit

Liquid limit:

The water content at which the soil damages from liquid to plastic state is known as liquid limit(LL). In this state, soil develop resistance to shear deformation.

Plastic limit:

The water content which brings soil from the plastic state to the semi-solid state is known as the plastic limit(PL). In this state, soil just fails to behave plastically.

Shrinkage limit:

The water content at which the soil changes from the semi-solid state to solid state is known as the shrinkage limit(SL).

Significance of Consistency Limits:

As the actual behavior of a soil depends upon its natural structure, the consistency limits do not give complete information about the in-situ soils. But these parameters are of great practical use as index properties of fine-grained soils. The index properties of such soils are related to the engineering properties as below:

1. Decrease in particle size increase liquid and plastic limits. But liquid limit increases at a greater rate resulting the rapid rate of increment in plasticity index.

2. Sandy soils, rather abruptly, changes from the liquid state to the semi- solid. These soil classified as non-plastic (NP).

Example: Soil having liquid limit less than 20% are generally sands

3. If organic matter is added, plastic limit of a soil increase, without any significant increase in liquid limit, i.e. soil with high organic content have low plasticity index

4. The compressibility of a soil is indicated by liquid limit, i.e. the compressibility of a soil generally increases with an increase in liquid limit.

5. The percentage of clay-size fraction present in the soil is directly proportional to shrinkage index. It defines the amount of clay.

6. Types and amount of clay in a soil is defined by liquid and plastic limit. But, plasticity index depends mainly on amount of clay, i.e. the plasticity index of a soil is a measure of the amount of clay in soil.

7. Liquid limit with plasticity index defines information about the types of clay. Classification of the grained soil can be obtain from plasticity chart.

8. When two soil have equal liquid limits, it observed that when plasticity index increase the dry strength and toughness increases, but the permeability remain almost the same.

9. If two soil of equal plasticity index is compared, it is noticed that as liquid limit increases, the dry streangth and toughness decrease, but compressibility and prmeability increase.

10. High percentage of colloidal clay containing mineral montmorillonite is defined by high toughness index.


  1. The definition of liquid limit should be revised as follows to give a better meaning/
    It is the minimum water content up to which the soil remains in a liquid state and doesnot resit any shear stres. It marks a transition from liquid to plastic state Similarly olatic limit i the minmum water content upto which the soil remains plastic and thereore remouldable.Yt marks the transition from
    plastic to solid state.

    The increae in LL and OL for fine grained oils is due ti increased surface atra of particles.

    U think the diffrences in behaviour of oils could be mainly attriuted to theirpore size distribution with clay dominent soils shwing more nicropore than capillary pores that can be easily emptied and filled while it needs considerable time for the microores. A oil rich in micropores and with leser no. of capillary pores may show high PI. A predominently capillary pore dented soil may show very little PI.

  2. Your elaboration is very helpful.. but little bit careful during typing can make your explanation more understanding...